Magnetic Scanning of the Radiation Characteristic of a CRLH CPW Antenna Gheorghe Sajin, Stefan Simion, Florea Craciunoiu, Andrei Muller, Alina-Cristina Bunea

National Institute for Research and Development in Microtechnologies, Bucharest, Romania gheorghe.sajin@imt.ro

stefan.simion@yahoo.com

florea.craciunoiu@imt.ro

andrei.muller@imt.ro

alina.bunea@imt.ro

Abstract – The paper presents the results concerning the radiation characteristic scanning of a ferrite supported resonating CPW antenna based on CRLH (Composite Right / Left-Handed) transmission lines. The scanning effect is obtained by magnetically polarizing the ferrite substrate. Data obtained by simulation with a suitable microwave software indicate an azimuthal characteristic deviation of ± 9.780 at a working frequency f = 13 GHz for a variable magnetic biasing field between 0 T and 0.16 T. This result is in very good agreement with the experimental data showing a characteristic displacement of +120 … -100.

I. INTRODUCTION A possibility to obtain transmission media having

metamaterial (MM) characteristics is to use particular artificial transmission lines obtained from cascaded cells of interdigital capacitors and parallel connected short-ended microstrip line inductors. Such CRLH (Composite Right/Left-Handed) transmission lines are reported in [1]. The CRLH cell opened a new class of circuit topology for many devices such as coupled-line directional couplers [2] and various types of antennas [3], [4]. In the domain of antennas made on the basis of MM approach, a lot of contributions were produced, more recent being [5] - [9]. Concerning the angle scanning of a CRLH antenna characteristic, this ability was electronically achieved by modulating the capacitances of the structure by adjusting the (uniform) bias voltage applied to some varactor structures [10].

There are very few contributions found in literature concerning microwave and millimeter wave CRLH components supported on magnetically biased ferrite.

Recently, authors [11] compared four related CRLH leaky-wave antennas with dispersion controlled by an applied magnetic field for fixed frequency external tuning.

In this contribution we present the radiation characteristic scanning of a CRLH CPW zeroth order resonator antenna having a magnetically polarized ferrite as supporting substrate.

II. MODELING THE SCANNING OF THE RADIATION PATTERN The antenna shown in Fig. 1 (a) and (b) is an array of three

CRLH cells, each having a T circuit topology formed of two series connected CPW interdigital capacitors and two parallel

short-ended CPW transmission lines as inductors cf. [12]. The antenna layout is shown in Fig. 1 (a), while Fig 1 (b) shows a photo detail of the area around the junction between the CPW interdigital capacitors and the CPW inductive lines.

(a)

(b) Fig. 1. The antenna layout (a) and a photo detail of the area around the junction

between the CPW interdigital capacitors and CPW inductive lines.

The substrate used in this work was a polycrystalline garnet with the saturation magnetization 4πMs = 550 G, permittivity ε = 13.5 and resonance linewidth ΔH = 16.8 kA/m. The thickness of the ferrite substrate was 0.5 mm and the metalized surface was previously mirror polished. The antenna layout was analyzed and modeled using the IE3D – Zeland software for an externally applied biasing field Happl = 0 T, namely, for the ferrite substrate in unmagnetized state. For the layout of

978-1-4244-5795-3/10/$26.00 ©2010 IEEE 337

one of the CRLH cells the following results were obtained: the CPW inductive line length – 1.5 mm; the width of the CPW central conductor – 100 μm; the width of the CPW slot – 100 μm; the length of the interdigital capacitor at the end of antenna – 1 mm and 0.5 mm for the internal interdigital capacitor; the width of the metallic finger of the interdigital capacitor – 10 μm; the space between two fingers of the interdigital capacitor – 10 μm; the space between the interdigital capacitor and the ground planes of the CPW structure – 100 μm. The number of the metallic fingers of the interdigital capacitor is equal to 10.

For this layout, the elements of the CRLH equivalent circuit for the interdigital capacitor and for the short-ended inductive CPW lines are: LL = 0.55 nH, CL = 0.18 pF, LR = 0.3 nH and CR = 0.23 pF.

The whole layout of the zeroth-order resonating antenna consists of three such CRLH cells, each one having the above dimensions. An input CPW line of 4.5 mm length was used for the connection to the measurement system.

When the ferritic substrate is biased by a dc magnetic field (Happl) applied normally on the ferrite substrate, the permeability changes its values from unpolarized state. The effective permeability μeff of the magnetically biased ferrite substrate was computed by inserting the relations (1)…(7) in a suitable software, cf. [13].

'

2'K2'eff μ

−μ=μ (1)

where:

( )[ ]( )[ ] 22

L24

22122L

2122LLM1'

αωω+α+ω−ω

α−ω−ωωω+=μ (2)

( )[ ]( )[ ] 22

L24

22122L

2122LM'K

αωω+α+ω−ω

α+ω−ωωω= (3)

where:

MsM ω=γ (4)

LiH ω=γ (5)

α ≅ ΔH/2Hr (6)

where:

Hr = gyromagnetic resonance field; Ms = saturation magnetization of the ferrite substrate; Hi = internal magnetic field in the ferrite substrate. For the geometric shape of the ferrite substrate – a thin

wafer having the thickness much smaller than the other two

dimensions – with the biasing magnetic field applied normally on its surface, the internal magnetic field (Hi) is computed using the following equation:

Hi = Happl – Ms (7)

A preliminary evaluation shows that if the applied magnetic field changes from Happl = 0 T to Happl = 0,26 T, the effective permeability (μeff) of the ferrite substrate, computed with the previous relations, decreases from μeff ≅ 1 for Happl = 0 T to μeff = 0.921 for Happl = 0.26 T.

It must be noted that for this domain of the applied field the antenna structure works under the gyromagnetic resonance.

These values for μeff were used in the IE3D software. The modeling of the antenna frequency and return-loss was carried out for four values of the applied magnetic field. The return-loss and the frequency for each value of the magnetic biasing field are specified in Table I.

TABLE I CALCULATED ANTENNA FREQUENCY AND RETURN LOSSES ON A

MAGNETICALLY POLARISED FERRITE SUBSTRATE

Applied magnetic fied (T)

Effective permeability

(μeff)

Frequency (GHz)

Return loss (dB)

0 1 12.88 28 0.055 0.986 12.96 20.26 0.18 0.951 13.20 22.30 0.26 0.921 13.41 17.70

The conclusion of this preliminary evaluation is that the

resonating frequeny of the ferrite supported antenna changes with 530 MHz, from 12.88 GHz for Happl = 0 T to 13.41 GHz for Happl = 0.26 T if a biasing magnetic field is applied normally on the substrate.

As it was shown in a previous work [14] a frequency change of the CRLH antenna (on ferritic or non-ferritic substrate) implies a change of the maximum azimuthal amplitude of the radiation characteristic. Cf. [14], the correspondence between the frequency shift and the position of the maximum amplitude of the radiation characteristic is given in Table II.

TABLE II. CALCULATED CORRESPONDENCE BETWEEN THE FREQUENCY SHIFT AND THE

AZIMUTHAL POSITION OF THE ANTENNA RADIATION CHARACTERISTIC

Frequency Maximum amplitude angle 14,6 GHz θ = 200 14,3 GHz θ = 80 14,2 GHz θ = 00 12,3 GHz θ = -500

One may see that for a frequency shift of 2300 MHz the

scanning angle of the antenna characteristic is 700; that means a rate of about 30 / 100 MHz.

Two antenna structures were manufactured according to the

previously described design features.

338

The experimental data showed a frequency shift of 400 MHz for the antenna structure #1 and 450 MHz for the antenna structure #2, due to the variation of the effective permeability of the ferrite substrate. The measured frequency shifts are consistent with the calculated values for a particular magnetic biasing field, the decrease of the return loss values being also consistent with the calculated values.

In the experiments regarding radiation characteristic scanning, the intensity of the polarizing magnetic field used was Happl = ±0.16 T with the corresponding value of the effective permeability μeff ≅ 0.96. The frequency shift in this situation was ±326 MHz obtained as a result of the modelling process done with the IE3D Zeland software. For this frequency shift, data previously presented indicate a total calculated scanning of the antenna radiation characteristic of 9.780.

III. EXPERIMENTAL RESULTS The technological process for the antenna manufacturing

consists of a standard wet etching photolithography process of 500 Å Cr / 0.6 μm Au metallization obtained by evaporation on the mirror polished side of the ferrite wafer. A microscope photo showing the configuration of the interdigitated capacitors and part of the inductor lines is shown in Fig. 1 (b). The antenna active area is 3,9 mm×3,4 mm, smaller by ~30%, than a λ/2 patch antenna. Two antenna structures mounted on suitable high frequency test fixtures are shown in Fig. 2.

Fig. 2. Test fixtures supporting the fabricated antennas on a ferrite substrate.

In order to measure the radiation characteristic of the ferrite

supported antenna, a measurement system able to measure the power radiated by the antenna at various angles while applying a dc biasing magnetic field normally on the ferrite substrate was used. This magnetic field was obtained from an electromagnet with one mobile armature and its strength was measured by a teslameter with a Hall probe. This measuring setup is shown in Fig. 3 (a). A photo of the experimental setup is shown in Fig. 3 (b). The emitting antenna is the antenna on ferrite, fixed close to a mobile electromagnet armature. The electromagnetic radiation is received by a horn antenna having

the possibility to rotate around the emitting antenna, the angle of this rotation being read on a circular dial marked in degrees.

The Hall probe was put close to the fixed armature and does not appear in the photo in Fig. 3.

(a)

(b) Fig. 3. The measuring arrangement (a) and the antenna structure in the

measuring setup for measurements in the transverse plane θ - (b)

A. Measurement of the radiation characteristic in transverse plane (θ)

The measurements of the radiation lobe were made both in transverse and longitudinal planes. For the definition of these planes, see Fig. 4.

Fig. 4. The transverse (θ) and the longitudinal (ϕ) measurement planes

339

The experiments were made with biasing magnetic fields having the values: Happl = 0 T, Happl = 0.16 T and Happl = −0.16 T, the last value being obtained by the reversal of the current through the electromagnet.

The power measured by the receiving horn antenna at different angles, with and without magnetic biasing, normalized to the maximum value read for Happl = 0 T at the angle θ = 0°, is given in Table III.

TABLE III

THE RATE P/PMAX AT DIFFERENT TRANSVERSE ANGLES (θ) FOR THREE VALUES OF THE BIASING MAGNETIC FIELD

θ (0) Happl

0.16 T 0 T −0.16 T −35 0.15 0.1 0.15 −30 0.19 0.12 0.19 −25 0.14 0.15 0.14 −20 0.24 0.13 0.24 −15 0.33 0.29 0.81 −10 0.33 0.68 0.87 −5 0.44 0.87 0.77 0 0.72 1 0.72 5 0.81 0.91 0.7

10 0.91 0.81 0.64 15 0.91 0.47 0.55 20 0.7 0.22 0.47 25 0.4 0.2 0.4 30 0.27 0.21 0.27 35 0.11 0.21 0.11

P/Pmax=f(θ)

0

0.2

0.4

0.6

0.8

1

1.2

-40 -35 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35 40θ (degrees)

P/P

max

Happl=0 T

Happl= - 0.16 T

Happl= 0.16 T

Fig. 5 (a). Scanning of the radiation lobe of the ferrite supported antenna in cartesian representation for Happl=0T; 0.16 T and -0.16 T. The values were

rated to the absolute maximum value.

Scanning of the radiation characteristics for Happl=0 T; 0.16 T and -0.16 T, respectively

0.2

0.4

0.6

0.8

1

30

210

60

240

90

270

120

300

150

330

180 0

θ

P/P

max

Happl = 0 T

Happl = 0.16 THappl = -0.16 T

(deg)

*Normalized for the maximum value at Happl = 0 T

Fig. 5 (b). Scanning of the radiation lobe of the ferrite supported antenna in

polar representation. Values were rated to the absolute maximum value.

One may observe that, if the ferritic substrate supporting

the antenna is biased by a dc magnetic field Happl = 0.16 T, the maximum of the radiation characteristic is moved to the value θ ≅ 120. If the orientation of the biasing magnetic field is reversed, keeping the initial magnitude, Happl = −0.16 T, the maximum of the radiation characteristic is moved in the opposite direction to the approx. value θ ≅ −100. This scanning of the radiation characteristic is shown in Fig. 5 (a) in cartesian coordinates and Fig. 5 (b) in polar coordinates. In these representations the values were rated to the absolute maximum value at Happl = 0 T and θ = 00.

In addition to the scanning effect one may observe an increase of the attenuation of the emitted signal.

In order to better approximate the scanning angle, in Fig. 6 (a) and (b) we rated the measured values to the maximum value for each applied magnetic biasing field. Due to the relatively low number of measurements, a Spline function approximation was used. The smoothing factor was set so as to obtain, in the end, values smaller or equal to 1. After reaching an adequate fit for the graphic in a cartesian plotting system, the corresponding values for each degree were selected, therefore obtaining 71 values, compared to the 15 initial values. These values were then inserted as vectors in an adequate numerical processing program. Since the values need to be in radians for a polar plot, a conversion from degrees to radians was done. This procedure was executed for all the directivity curves, normalized at the absolute maximum value (for Happl = 0 T and θ = 00) and at the maximum value for each applied dc magnetic biasing field (Happl = 0.16 T and Happl = −0.16 T), representing them, in the end, on two separate figures.

340

P/Pmax=f(θ)

0

0.2

0.4

0.6

0.8

1

1.2

-40 -35 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35 40θ (degrees)

P/P

max

Happl=0 T

Happl= - 0.16 T

Happl= 0.16 T

Fig. 6 (a). Scanning the radiation lobe of the ferrite supported antenna in

cartesian representation. Values were rated to the maximum value for each applied magnetic biasing field.

Scanning of the radiation characteristics

for Happl=0 T; 0.16 T and -0.16 T, respectively

0.2

0.4

0.6

0.8

1

30

210

60

240

90

270

120

300

150

330

180 0

θ

P/P

max

Happl = 0 T

Happl = 0.16 THappl = -0.16 T

(deg)

*Normalized for the maximum value at each applied field

Fig. 6 (b). Scanning the radiation lobe of the ferrite supported antenna in polar representation. Values were rated to the maximum value for each applied

magnetic biasing field.

B. Measurement of the radiation characteristic in longitudinal plane (ϕ)

In order to complete the characterization of the antenna’s radiation capability, measurements were also made in the longitudinal antenna plane (ϕ) – see Fig. 4. Measurements were done for two antenna structures at 10 biasing magnetic field values but only the most representative values were retained. The measuring setup shown in Fig. 3 (b), was slightly modified and the measuring angle was in forward direction.

The results for the two antenna structures (#1 and #2) are shown in Fig. 7 (a) and (b) where the radiated power at different angles in the (ϕ) plane in the forward direction for three values of the biasing magnetic field, were plotted. All the

radiated power values were rated at the value P(0 T, 0 deg) meaning the measured value of the radiated power in absence of the biasing magnetic field (Happl = 0 T) in a point located on the normal line on the antenna center at ϕ = 00 (see Fig 4). All the other measured values were rated to this value.

From Fig. 7 (a) and (b) one may find that the maximum radiated power for both antenna structures occurs at an angle ϕ ≅ 50, regardless of the magnetic biasing of the ferritic substrate. The radiated power gradually decreases, as it can be seen in Fig. 7. The graph was limited to the angle value of ϕ ≅ 30° because beyond this value it becomes insignificant.

While applying a magnetic polarization, with field values between Happl = 0 T and Happl = 0.1 T, the radiated power demonstrates an increase, which is more evident for antenna structure #1.

Scanning Radiation Antenna 1 - P/P(0 T, 0 deg)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 5 10 15 20 25 30Degrees in (ϕ) plane

P/P

(0 T

, 0 d

eg)

0 T0.03 T0.09 T

Fig. 7 (a). Radiated power in (ϕ) plane for antenna structure #1

Scanning Radiation Antenna 2 - P/P(0 T, 0 deg)

0

0.2

0.4

0.6

0.8

1

1.2

0 5 10 15 20 25 30

Degrees in (ϕ) plane

P/P

(0 T

, 0 d

eg)

0 T0.03 T0.09 T

Fig. 7 (b). Radiated power in (ϕ) plane for antenna structure #2

The rate between the current power value and the value

P(0 T, 0 deg) is almost 1.4 for antenna structure #1 and about 1.1 for antenna structure #2. This radiated power increase may be found for all the measurement points along the graph, the whole curve being “raised up”, but the maximum remains at the angle ϕ ≅ 50. Therefore, for the radiation characteristic along the (ϕ) plane of the antenna, no scanning effect was observed.

341

IV. CONCLUSIONS We present a CRLH CPW zeroth-order antenna on ferrite

substrate having the capability to scan the radiation characteristics following the magnetic ferrite biasing. The antenna was analyzed using a full-wave electromagnetic software. The calculated data indicate a scanning possibility of θ = ±9.780 in the θ plane for a magnetic biasing field Happl = ±0.16 T.

Experimental data shows a radiation characteristic scanning ability between the angles θ ≅ 120 for Happl = 0.16 T and θ ≅ −100 for Happl = −0.16 T. The experimental results are consistent with the calculated values.

In addition to the scanning effect one may see an increase of the attenuation of the emitted signal in the (θ) plane of the antenna.

Concerning the antenna characteristic in the forward direction in the (ϕ) plane, we did not find a scanning effect, but an increase of the radiated power consecutive to magnetically biasing the ferrite substrate was observed.

REFERENCES [1] C. Caloz, T. Itoh, “Electromagnetic metamaterials: transmission line theory and microwave applications”, John Wiley & Sons, Inc., 2006. [2] C. Caloz, A. Sanada, T. Itoh, “A novel composite right-/left-handed coupled-line directional coupler with arbitrary coupling level and broad bandwidth”, IEEE Trans. on Microwave Theory and Techniques, vol. 52, no. 3, pp. 980-992, March 2004. [3] A. Sanada, M. Kimura, I. Awai, S. Caloz, T. Itoh, “A planar zeroth-order resonator antenna using a left-handed transmission line”, Proc. of the 34th European Microwave Conference, pp. 1341-1344, Amsterdam, 2004.

[4] Simion S., Marcelli R., Sajin G, “Small size CPW silicon resonating antenna based on transmission-line meta-material approach”, Electronics Letters, Vol.43, No.17, pp.908 – 909, ISSN 0093-5914. [5] Toru Iwasaki, Hirokazu Kamoda, Thomas Derham, Takao Kuki, “A Composite Right/Left-Handed Rectangular Waveguide with Tilted Corrugations for Millimeter-wave Frequency Scanning Antenna”, Proc. of the 38th EuMC, pp. 563-566, Amsterdam, 2008. [6] Kohei Mori, Tatsuo Itoh, ‘‘Distributed Amplifier with CRLH-Transmission Line Leaky Wave Antenna’’, Proc. of the 38th European Microwave Conference, pp. 686-689, Amsterdam, 2008. [7] Cheng-Chi Yu et al., “A compact antenna based on metamaterial for WiMAX”, Proc. of Asia-Pacific Microwave Conference, Paper J2-05, Hong Kong, 2008. [8] Seongmin Pyo et al., “A metamaterial-based symmetrical periodic antenna with effiency enhancement”, Proc. of Asia-Pacific Microwave Conference, Paper A3-49, Hong Kong, 2008. [9] Chen-Jung Lee, Maha Achour, Ajay Gummalla, “Compact metamaterial high isolation MIMO antenna subsystem”, Proc. of Asia-Pacific Microwave Conference, Paper A2-27, Hong Kong, 2008. [10] S. Lim, C. Caloz, T. Itoh, “Metamaterial-based electronically controlled transmission-line structure as a novel leaky-wave antenna with tunable radiation angle beamwidth”, IEEE Trans. on Microwave Theory and Techniques, vol. 52, no. 12, pp. 2678-2690, Dec. 2004. [11] Toshiro Kodera, Christophe Caloz, “Comparison of various ferrite-loaded CRLH leaky-wave antenna structures”, Proc. of Asia-Pacific Microwave Conference, Hong Kong, 2008, Paper J3-06. [12] A.Lai, C.Caloz, “Composite Right / Left – Handed Transmission Line Metamaterials”, IEEE Microwave Magazine, Sept. 2004, pp. 34 – 50. [13] B. Lax and K. J. Button, Microwave Ferrites and Ferrimagnetics, USA: McGraw-Hill Book Comp., Inc., 1962. [14] G. Sajin, S. Simion, F. Craciunoiu, R. Marcelli, “CPW Silicon Zeroth-Order Resonance Antenna Using Composite Right/Left-Handed Transmission Lines”, Proceedings of the Mediterranean Microwave Symposium, MMS 2007, Budapest, Hungary, 14 – 16 May 2007, pp. 65 – 68.

342